1. Field of the Invention
The invention relates to reuse of a gas discharged from a vacuum pump. More particularly, the invention relates to reuse of a gas discharged from a vacuum pump which operates using a gas for purposes of shaft sealing, improvement of a degree of vacuum, prevention of formation of reaction by-products, inhibition of corrosion, elongation of the life of device, and the like.
2. Description of the Related Art
As an example, in a dry etching step and a chamber cleaning step for a CVD apparatus for a process for manufacture of semiconductor devices, perfluorocompounds (PFCs), such as methane tetrafluoride (CF.sub.4), ethane hexafluoride (C.sub.2 F.sub.6), and nitrogen trifluoride (NF.sub.3), are used to etch polysilicon (poly-Si), silicon oxide (SiO.sub.2) and the like.
In such steps, a chamber is evacuated by a vacuum pump to a reduced pressure of, for example, the order of 13.3 to 66.5 pascals, and subsequently, a PFC gas is introduced to the chamber to produce a plasma and dry-etch SiO.sub.2 or the like. The details of the mechanism of an etching process are not known, but it is presumed that in a chamber, a radical molecule generated from a PFC excited by a high-frequency field reacts with SiO.sub.2 or the like present on the surface of a treated substrate or deposited on walls and the like inside the chamber, to thereby proceed with etching, which produces gas components such as SiF.sub.4.
In such an etching step, all of the PFC introduced to a chamber is not decomposed and the undecomposed gas is discharged from the chamber by a vacuum pump. As the vacuum pump, such a pump as a dry pump, a mechanical booster pump, or a turbo molecular pump is used, in general. Nitrogen gas is introduced to the vacuum pump, as an inert gas separate from the gas emitted from the chamber, for purposes of shaft sealing, improvement in the degree of vacuum, prevention of formation of reaction by-products (in the case where the gas from the chamber is reactive, products resulted from reactions can be deposited inside the vacuum pump), inhibition of corrosion, elongation of the life of the device, and the like, and is discharged from the vacuum pump along with the gas emitted from the chamber.
Exhaust discharged from a vacuum pump contains gases such as SiF.sub.4 formed by the reaction of radical molecules generated from a PFC with SiO.sub.2 or the like, and gases formed by decomposition of the PFC by a high-frequency field. Since these gas components include those having a high corrosiveness and high toxicity, the exhaust from a vacuum pump is, in general, modified to have no toxicity by a detoxificating unit prior to being emitted to the atmosphere. As the detoxificating unit, a device which is filled with an adsorbent such as an activated carbon, activated alumina or molecular sieve, or a chemically reactive agent produced by loading an alkaline agent or the like on activated carbon or alumina, is generally used.
Nevertheless, it has recently been deemed to be important to prevent global warming, and the exhaust of gases such as PFCs, having a high global warming potential (GWP), tends to be restricted. To cope with such situation, techniques in which gases having a high GWP in the exhaust from a vacuum pump are decomposed and detoxificated prior to being emitted to the atmosphere, as referred to above, or the exhaust is compressed and gaseous impurities are separated from the compressed exhaust to recover PFCs, have been developed.
As the techniques for decomposing PFCs in an exhaust, methods such as thermal cracking, catalytic thermal cracking, and combustion cracking have been developed. In these processes, PFCs having more carbon atoms are gradually decomposed to PFCs having fewer carbon atoms. For example, when plasma discharge was established in a chamber under the condition of C.sub.2 F.sub.6 flow rate of 0.1 liter per minute, and the exhaust from the chamber was discharged by a vacuum pump to which N.sub.2 was introduced at a flow rate of 28 liters per minute, it was observed that the exhaust at the outlet of the vacuum pump contained 0.01% CF.sub.4, 0.12% C.sub.2 F.sub.6, 0.01% CH.sub.2 F.sub.2, and in addition, trace amounts of SiF.sub.4, HF, F.sub.2 and so forth. When the exhaust was further treated in a PFC decomposition unit of thermal cracking type, it was observed that the discharged N.sub.2 contained 0.06% CF.sub.4 0.09% CO, and 0.04% CO.sub.2, which reveals that the decomposition in the PFC decomposition unit resulted in formation of CF.sub.4.
However, since CF.sub.4 has a very large GWP 100 of 6,500, it is required to further decompose CF.sub.4 into carbon dioxide and fluorine, and change the resultant fluorine to a fixed form prior to removal.
To decompose CF.sub.4 in an exhaust, an unreasonable amount of energy is required, which may be unfavorable in the prevention of global warming, in terms of the carbon dioxide generated during the creation of the energy.
Further, in prior gas discharging or evacuating systems, a large amount of nitrogen is introduced to a vacuum pump, and is incorporated in gas emitted from a chamber, as referred to above. In this case, a decomposing unit consumes energy also for the diluted emitted gas, resulting in an increase in energy loss.
Methods of recovering and reusing exhaust containing PFCs have been developed to prevent global warming. In these methods, since there is dilution of the exhaust by nitrogen introduced into a vacuum pump, complicated processes and much energy are required to separate the PFCs and nitrogen in the exhaust.